Is this safe? Controlling a live load using using the MCC

[Jraef]

I don't think it is dangerous so much as it may result in some minor reduction in equipment life the big problem imo is the time wasted in doing this kind of thing and what loss of production results.

I agree. There is no rule against it, other than common sense. Disconnects and breakers built to NEMA standards are required to handle starting duty of motors. The thing is, the test durability requirements are not as tough for them as it is for contactors. Knife switches like what is shown must, per NEMA KS-1 standard, be able to switch at their RATED motor load at least 1,200 times (UL requires only 1,000 operations). But although the NEMA standards for contactors don't specify a number of operations, the general design is for 1 million electrical operations under rated load, 10 million mechanical. Bottom line, switching something on and off using the disconnect is not a great idea because you will severely decrease their service life and they are not simple to replace (in an MCC bucket), but is not something expressly forbidden.

A better way to temporarily get around interlocking issues (which are there for a REASON by the way) would be to add a "Jog" momentary push button that just ignores all other control paths and goes straight to the contactor coil.

 

[garbo]

Hi All.
Great forum! I was trying to find an answer to an issue today and found your forum.
I work as a mechanic for a company that uses belt conveyors to move product.
When the conveyors were overwhelmed today with a lot of product, I saw building supervisors (who are neither mechanics or electricians) cycling off MCC bucket disconnects to control different sections of the belts.
It was to my understanding that this is unsafe. These buckets control 480v, 3 hp motors. I am in belief that the control stations should be utilized, followed by using the regular disconnects as a second choice, followed last by using the knife switch at the actual mcc.
I know that our motor controls centers haven't been tested/rated so they do not have any type of rating info instructing what level of ppe to wear. Supervisors here just wear street clothing.

I know why they do this. If they use the control station, it shuts down All of the belts. If they use a disconnect, they can cycle off sections and let product run out from a jammed area.
Nonetheless, it seemed like the most dangerous choice was made to do this at the bucket versus a disconnect.
From what I've read, it looks like an arc flash Can also take place at a disconnect but is it More likely to occur at a bucket?

To be clear, they are stopping and starting loaded belts abruptly by opening and closing disconnects manually, engaging and re-engaging the live load to the motor.

It's not my area of expertise so I definitely wanted to ask you all for some help.
If I'm wrong or over worried, no problem. If they are doing something that will surely lead to an injury, I'd like to come up with a plan to better help them going forward.

Thanks!

I am a retired sparky with 50 great enjoyable years getting paid great money to play with wires. Not sure if it's against any codes but best practice is to only use a stop button on control panel to turn motor loads in & off. A lot cheaper to only replace contacts in a quality NEMA starter then replace an expensive circuit breaker. Have come across some old MCC'S that circuit breaker were obsolete. Same thing with safety switches. A few years ago I LOTTO old typical junk ge 600 amp circuit 600 volt breakers in the winter for tech working on chillers. Even though I turned them with no load ( chillers were off for the winter ) neither would turn back on. Took 3 weeks and around 5 grand each to replace. Read years ago in great EC&M magazine that cheap 15 & 20 amp residential circuit breakers are tested for hundreds of operations while the 100 amp main service circuit breaker is only tested for way less # of on off operations. Had a contractor blow up two VFD'S on cooling tower fans he was working on. Each of the 4 cooling tower fans had 100 amp outdoor safety switches w/o a NC micro switch that is opened before switch to open up M1 Contactor feeding power to drive. Contractor was too lazy to walk down two floors to properly start & stop motors at drive control panel. So tell people not to ever except in an emergency to use a bucket handle to turn power on or off. Easy job to install a two position in off selector on front of MCC'S. Years ago we installed triptronics Controls in screw & belted conveyors. When a conveyor started to draw too much current due to too much weight in conveyor or on a belt it would shut off feed conveyor and a few second timer would time out and restart conveyors.

 

[ptonsparky]

This would have been a whole lot cheaper to have done properly even a couple years ago.

 

[Jraef]

This would have been a whole lot cheaper to have done properly even a couple years ago.

Isn’t that always the case?

 

[paulengr]

Put simply, breakers are designed to operate under thousands of amps if a short circuit failure occurs. But they only have to survive a hundred cycles under load to pass UL. Switching with no load is fine. Same with disconnects.

Starters are designed for thousands of cycles under load even a dozen times per hour but will melt and be destroyed if they attempt to open in a short circuit.

Breakers that fail under load can turn into an arc flash. The manufacturers instructions say to follow NEMA AB-4 which says to visually inspect for damage every time it trips or is forced open under load. So it should be no problem as long as someone qualified to inspect opens the door to look after EVERY trip or cycle.

Also check the overloads. I’m surprised they haven’t shut this stupidness down to protect the motors from burning up. I’ll bet they’re oversized.

No really you have a choice in design between lots of lower power switching or a few big cycles. Breakers and starters can’t do both. The only exception is a special SWD (switching duty) breaker but these are rare.

So you will wear out breakers prematurely and have the potential for a serious burn injury doing this. That’s why you don’t do it.

Just put JOA switches in at each conveyor. That’s Jog I Off Auto. The jog is spring return which prevents them from running everything in manual. And have them watch arc flash training videos.

 

[ATSman]

I am commenting here to make aware of the dangers of working around large MCC's due to the Available Fault Current needed to supply power to multiple motors. I have heard many stories of electricians getting hurt and even killed by removing and installing buckets with the the MCCs energized. I have great respect for the power they have and hope to impart that knowledge in the name of safety. In my younger days I was blown off a ladder testing motor overloads due to arc flash and am lucky to be alive to tell about it. It was a stupid mistake I made and this is a first hand story told in the attached file.

 

[paulengr]

I am commenting here to make aware of the dangers of working around large MCC's due to the Available Fault Current needed to supply power to multiple motors. I have heard many stories of electricians getting hurt and even killed by removing and installing buckets with the the MCCs energized. I have great respect for the power they have and hope to impart that knowledge in the name of safety. In my younger days I was blown off a ladder testing motor overloads due to arc flash and am lucky to be alive to tell about it. It was a stupid mistake I made and this is a first hand story told in the attached file.

You have apparently never seen old Square D MCC buckets with built in disconnects nor newer MCCs built to the new specs with this. So you can’t make blanket comments about MCC buckets. Same with saying ALL switchgear can blow up during racking.

Especially when that wasn’t part of the conversation.

And you may not realize it but arc flash energy generally INCREASES with lower fault current because the inverse time nature of fuses and breakers increases faster than the linear decrease in current and roughly quadratic decrease in arc power (I-squared R) while time increases exponentially.

The threat with ladder work is being startled. All the arc flash videos on YouTube have a delay of about 350 milliseconds before the victim is “blown away” but arc blast occurs at about 2-3 cycles (50 milliseconds). Why the delay? That’s how long it takes for neurons to fire in the cerebellum and trigger muscles to twitch in a fight or flight reaction. Many “electrical injuries” are from falling off ladders or out of manlifts with little or no burn injuries ir shock.

 

[kwired]

You have apparently never seen old Square D MCC buckets with built in disconnects nor newer MCCs built to the new specs with this. So you can’t make blanket comments about MCC buckets. Same with saying ALL switchgear can blow up during racking.

Especially when that wasn’t part of the conversation.

And you may not realize it but arc flash energy generally INCREASES with lower fault current because the inverse time nature of fuses and breakers increases faster than the linear decrease in current and roughly quadratic decrease in arc power (I-squared R) while time increases exponentially.

The threat with ladder work is being startled. All the arc flash videos on YouTube have a delay of about 350 milliseconds before the victim is “blown away” but arc blast occurs at about 2-3 cycles (50 milliseconds). Why the delay? That’s how long it takes for neurons to fire in the cerebellum and trigger muscles to twitch in a fight or flight reaction. Many “electrical injuries” are from falling off ladders or out of manlifts with little or no burn injuries ir shock.

When I was younger I was drilling a hole in back of bucket to tap and then screws for mounting a control relay, timer, etc. (can't recall anymore exactly what type of device it was) in a MCC bucket. thought having that bucket disconnect off was pretty safe thing, never gave a thought to the bus plug assembly on back side of that back wall. Luckily about most that happened was I maybe needed clean shorts. The GFP main did trip and of course everything running off the MCC shut down. I was seeing stars for a while afterwards, but could potentially been much worse.

The good, bad and ugly about this is that I probably learned a lot more from this incident than I would have from watching safety videos.

 

[ATSman]

You have apparently never seen old Square D MCC buckets with built in disconnects nor newer MCCs built to the new specs with this. So you can’t make blanket comments about MCC buckets. Same with saying ALL switchgear can blow up during racking.

Especially when that wasn’t part of the conversation.

And you may not realize it but arc flash energy generally INCREASES with lower fault current because the inverse time nature of fuses and breakers increases faster than the linear decrease in current and roughly quadratic decrease in arc power (I-squared R) while time increases exponentially.

The threat with ladder work is being startled. All the arc flash videos on YouTube have a delay of about 350 milliseconds before the victim is “blown away” but arc blast occurs at about 2-3 cycles (50 milliseconds). Why the delay? That’s how long it takes for neurons to fire in the cerebellum and trigger muscles to twitch in a fight or flight reaction. Many “electrical injuries” are from falling off ladders or out of manlifts with little or no burn injuries ir shock.

"You have apparently never seen old Square D MCC buckets with built in disconnects nor newer MCCs built to the new specs with this."
Not sure what you meant by that statement. All the MCC's I've work on had a disconnecting means that only removes power inside the bucket. But the fact that they are designed for the buckets to be removed while the main bus is energized does not lessen the safety risk of what can happen if something goes wrong.

"Same with saying ALL switchgear can blow up during racking."
Please show me in my post where I made that statement?!? If you practiced re-reading a post maybe you won't go off half-cocked.
I was just relating an incident that nearly took my life due to ignorance with the hopes it could save some one's, when working around high energy
equipment. Nothing more and nothing less.
And as Kwired mentioned, there is nothing like first hand experience.

 

[Jraef]

"You have apparently never seen old Square D MCC buckets with built in disconnects nor newer MCCs built to the new specs with this."
Not sure what you meant by that statement. All the MCC's I've work on had a disconnecting means that only removes power inside the bucket. But the fact that they are designed for the buckets to be removed while the main bus is energized does not lessen the safety risk of what can happen if something goes wrong.

Some newer MCC designs have an option that allows you to pull the stabs off of the bus with the unit door still closed. Eaton Flashgard is like that, A-B has an option called Secure Connect that you can add to any bucket. I think maybe Square D has something now but I haven’t come across a Sq. D MCC in a long time.